The invention relates to electrical and electronic circuits and techniques for making accurate measurements of resistance and on a continuing basis; being more particularly, though not exclusively, concerned with such measurement wherein sensed physical parameters are converted or transduced in a sensing resistance for measurement, as for remotely monitoring construction or other equipment (for example, for fuel gauge measurements) and where it is necessary to measure and sense resistance without having full physical access to the sense resistor (as of the fuel gauge or the like).
While, as more particularly explained hereinafter, the invention is of much more general utility, the above illustrative example of remotely monitoring construction or other equipment is an important application of the technique underlying the invention and will be hereinafter discussed as an exemplary, though not exclusive, use. In copending U.S. patent application Ser. No. 09/416,604, filed Oct. 12, 1999, and of common assignee herewith, such systems are described for remotely keeping track of the location and the operating conditions of fleets of vehicular construction equipment through each locally sensing such conditions or parameters as engine fuel level, engine run hours and the like, and supplying information as to such by radio communications links to a remote monitoring center or station, thereby enabling the fleet manager remotely to monitor and maintain the operation of the fleet and to improve efficiency of equipment utilization through such appropriately processed and displayed information.
In many data acquisition applications, a sensor transduces or converts a physical quantity being sensed into an electrical resistance, which can be measured by electronic means. This is done in many fuel level sensors used in heavy construction equipment and the like, wherein a resistance controls the amount of current passing between the equipment ground and the fuel gauge, causing a proportional amount of deflection of the fuel gauge needle.
Fuel sensors, for example, are basically variable resistors, and they cover a broad range of resistance depending upon the make and model. There is difficulty involved in measuring such a wide range and, in particular, making accurate measurements on those fuel sensors which have a very low resistance and operate in an environment with considerable electronic noise produced from the vehicular equipment engine.
In accordance with the present invention, these problems in accurate resistance measurements of variable sensing low-value resistors, are admirably solved through the use of a programmable current source operating with a known reference or standard resistor for calibration. Basically, two different currents are measured on the known reference resistor, and then two different currents on the sensing resistor, and then appropriate calculations are made, including appropriate subtraction and ratios hereinafter detailed, to enable eliminating all the above-described sources of error and over a wide range of sensing resistor values, including low resistances, and over wide temperature ranges xe2x80x9435xc2x0 to 85xc2x0 C., more or lessxe2x80x94in the real world of construction equipment fleets, and always with highly accurate readings.
The conventional way to measure resistance R, of course, is to put a known current i through the resistance and measure the voltage V across it; the two being related simply by Ohm""s Law, V=iR. When, however, the resistance belongs to a foreign or remote system being monitored, a number of practical limitations intervene, which it is the purpose of the present invention to address and to mitigate, as will now be explained.
One of these practical limitations often resides in the lack of the proper common ground reference. In an exemplary system for monitoring construction equipment remotely, as an illustration, it is necessary to measure such a sensed resistance without having full physical access to both terminals of the sense resistor. In this example, it is convenient to connect to the sensor at the gauge; but the ground terminal of the sense resistor is not accessible, lying deep within the vehicle near the fuel tank. The measuring system will connect to the equipment chassis to establish a ground reference, but that ground reference may well not be the same as that of the sensorxe2x80x94and there may indeed thus be current flowing between the two, causing a potential difference due to ground loops in the equipment. As a result, when a known current i is sent through the resistor R, the voltage to ground that is measured will not be V=iR, as expected from Ohm""s Law, but rather V=iR+Vo, where Vo is an unknowable potential difference between the two ground reference points above mentioned.
A second limitation may reside in temperature-dependent voltage offsets in the measuring circuitry, arising from the practical limitations of components in such measuring circuitry; in particular, operational amplifiers used to convert the range of voltages being measured to a convenient part of the operating range of the measuring circuitry. These effects vary from component to component, and with temperature for a single component; therefore, they can only be taken into account by the expensive technique of making detailed measurements on each manufactured instance of the measuring circuitry.
Component value inaccuracies in measurement circuitry can also produce multiplicative errors. The gain of an operational amplifier, for example, may differ by a small percentage from its nominal value due to allowable errors in its own construction and the resistors used to set the gain. For similar reasons, the current actually generated may be in error by a small percentage. In such event, the measured voltage V, instead of the above, will have a value V={1+e}iR+Voff, where i is now the intended current, e is an unknowable but small error, and Voff (hopefully small) is also unknowable, consisting of a sum of components of the original Vo which are subject to any of, none, some, or all of the multiplicative errors in the system.
Still another factor inhibiting accurate resistance measurements, particularly in the setting of the above exemplary application of the invention, is the effect of noise and interference. All circuitry is, of course, subject to interference and noise; some rides on the external signal being monitored, some is induced from currents in adjacent circuits, and some is generated by thermal processes within components of the circuit itself. Components such as the before-mentioned operational amplifiers, moreover, have input voltage offset specifications and do not respond linearly to voltages below the specification. Noise can be partially mitigated by processing many measurements taken over time (filtering): but not if it gets distorted because the total voltage is driven below the amplifier specification.
The measuring circuit, therefore, has a voltage floor, below which it cannot measure with any accuracy, which is herein termed xe2x80x9ca noise floorxe2x80x9d, for simplicity. If the range of values of R to be measured includes values too low (relative to the current source), V might well be below the noise floor. It is then necessary to introduce an offset resistor ROFF in series with R, where ROFF is chosen such that the voltage across ROFF+R due to current i will always exceed the noise floor. R+ROFF is then measured instead of R, and ROFF is subtracted later at the end. Any deviation of the component ROFF from its nominal value will, however, be attributed to R, so that when R is smaller than ROFF, it will be subject to a percentage error much larger than the tolerance of ROFF (by a factor R/ROFF). If, for example, ROFF is nominally 30 ohms and has a 1% tolerance, and R is 2 ohms, an 0.3 ohm overage in ROFF might occur and cause R to be calculated as 1.7 ohms, a 15% error.
An additional limitation involves the dynamic range of the voltage measuring circuits. Such voltage measuring circuits have maximum voltages that they can measure, limited in some way by their own system operating voltages. If the R being measured gets large enough, the limit may be exceeded. While it is easy to scale down voltages, voltages corresponding to low values of R may well go below the noise floor, with the deleterious error consequences above discussed. The possibility of an intolerable limitation on the range of R, thus also needs to be addressed.
While lack of a common ground between the sensor resistor and the equipment chassis was previously addressed, there may also be other common ground problems. It may be, as another illustration, that the sensor and the measuring system power source do not share a common ground. This is often the case, for example, when the construction equipment is outfitted with a master cut off switch that disconnects the equipment chassis ground from the negative battery terminal, from which the sensor system is powered. The portion of the circuitry in electrical contact with the sensor then needs to be electrically isolated from the rest, which may include the processing xe2x80x9cbrainsxe2x80x9d of the device. The present invention is designed to address every one of the above limitations on enabling accurate resistance measurements, particularly in remote equipment monitoring applications and the like, as will later be more fully explained.
It is a primary object of the invention, accordingly, to provide a new and improved method of and apparatus for accurate resistance measurements and with the aid of a novel programmable current source that remarkably enables obviation of all of the prior art measurement limitations earlier discussed.
A further object is to provide such a novel resistance measurement technique that is particularly, but not exclusively, useful where both terminals of the sensor resistor are not readily available to the measurement circuit, as in, for example, the remote monitoring of sensing resistors (as associated, as an illustration, with fuel gauge measurements) of vehicular construction equipment, or in similar applications.
Other and further objects will be explained hereinafter and are more particularly delineated in the appended claims.
In summary, however, from one of its important aspects, the invention embraces a method of accurately measuring the varying resistance of a sensing resistor, that comprises, alternately applying pairs of different currents to each of a sensing resistor and a standard reference resistor to develop corresponding pairs of different voltages; subtracting the different voltages of the respective pairs of different voltages; subtracting the different currents of the respective pairs of currents applied to the respective resistors; and taking the ratio of the subtracted pairs of different voltages each divided by the corresponding subtracted differences in the currents of the respective pairs of currents.
Preferred and best mode embodiments and designs are hereinafter explained in detail.